The invention relates to a method for producing welded tubes from steel
In the following welded tubes mean helical seam tubes, so called spiral welded tubes, as well as longitudinal seam welded tubes.
For transporting water, oil and gas usually submerged arc welding (UP), high frequency induction welded (HFI) or electric resistances welded (ERW) conducting tubes are used, which are preferably produced for hot strip or from plates in thicknesses of about 10 to more than 25 mm.
Helical seam tubes are usually UP welded, while longitudinally seam welded tubes beside UP welded can also be HFI or ERW welded. However increasingly laser or laser hybrid welding consisting of a combination of laser and protective gas welding, is also used for such tubes.
In the longitudinal seam welded large tubes, which are welded with the UP method, the method referred to in the art as UOE is the most commonly used method. In this method in a first step the edges of an even steel sheet are bent. The subsequent die forms the steel sheet with a round die into a U-shape (U-pressing tool), which is subsequently pressed on an O-pressing tool with two closing dies to a round open seam tube. Subsequent to the forming of the steel sheet onto an open seam tube the latter is finished welded in a second step by means of UP welding. Because in many cases the thus produced tubes after internal and external welding do not yet meet the requirements regarding diameter, roundness and straightness they are calibrated (expansion) by cold expansion. The thus produced tubes are provided with a coating depending on the demands on corrosion resistance and are welded to pipelines at the construction site and are for example used for the oil and gas transport.
For helical seam tubes it is known from the brochure “spirally welded large tubes—product information ” (Salzgitter Mannesmann large tube 3/08), to align the hot strip after uncoiling from a coil, to form the hot strip helically to a open seam tube in a forming device, and after forming the strip is welded in a two step process to a tube.
For this the strip is formed in a forming unit or a tube forming machine to a tube. The forming unit consists of a 3 roller bar bending system with an outer roller support cage and a so-called offset roller. With the height adjustable offset roller a possible strip edge offset of the open seam tube can be compensated, In this manufacturing process which is known as HTS-method” (Helical Seam Two Step) the strip edges of the open seam tube are welded in a first step by means of protective gas tack welding at high welding speeds of up to 15 m/min, wherein the strip edges are only partially connected to each other.
The tube diameter is influenced by the feed angle of the strip into the forming unit and by the strip width of the used starting material. By means of the height adjustable offset roller the diameter of the rube can also be influenced.
The final welding with a complete welding oft the strip edges with an inner and outer seam occurs subsequently in a second step on separate welding stands by means of UP welding.
The advantage compared to the conventional one-step method in which also the submerged arc welded seams are directly produced in the tube forming machine and welding of the tube is thus completed in one step, are that the high speed of the tack welding achieves a higher performance of the tube forming machine.
After the welding, the spiral welded tubes are subjected to a water pressure test for testing tightness, and are subsequently tested in multiple stations whether they meet the quality standards and are then prepared for shipping.
Depending on the demands on the corrosion resistance the longitudinally or spiral welded tubes can also be provided with a coating.
In particular under conditions in which pipelines are installed offshore and are subjected to very high external pressure, the tubes have to meet ever increasing demands regarding the roundness tolerances due to the increasing demands on for example collapse resistance. However, not only the geometry (roundness, straightness) but also the material properties such as strength, tenacity etc., significantly influence the performance characteristics of the tubes.
There is a demand for a high constancy of the tube properties regarding geometry and material and also the tube cross-section and length of a tube as well as between tubes compared to each other. The uniformity of these demanded properties is influenced by many factors during the entire production process.
These factors are on one hand the material properties of the starting material (strip or metal sheet) such as for example strength, tenacity microstructure, texture, internal tension sate etc., which are to be as uniform and homogenous as possible along the length and width. On the other hand these factors also involve the constancy of the selected process parameters for example during forming and in the subsequent manufacturing steps.
From GB 2 027 373 A it is known that the inherent stress condition of the strip has an influence in the uniformity of the tube geometry (ovality, straightness etc.). Determination of the inherent stress condition by itself however is not sufficient to characterize the quality of the finished tube because the further influencing factors remain unaccounted for.
Within the scope of the quality control therefore many integrated destructive and different nondestructive tests are carried out in the manufacturing process of the welded tubes to ensure the qualitative demands on the finished tube.
The quality-ensuring measures are comparable in longitudinally or helical seam welded tubes.
The material characteristic values of the starting material, i.e., the strip or metal sheet material, are usually determined by random sampling by means of destructive testing methods such as for example pull test or notch impact test and beside the geometric values (length width thickness) are taken into account for adjusting eh manufacturing parameters such as for example forming of the strip or metal sheet, welding parameters or subsequent alignment processes.
It is for example known that the mechanical material properties or the inherent stress condition due to different cooling conditions during rolling of the strips and metal sheets can vary over their width and length. When the properties of the strip or metal sheet are known the process parameters can correspondingly be adjusted.
With the known methods of the destructive material testing, however only a local and random testing and control of the strip and metal sheet properties and with this only a limited assessment of the effect on a possible required adjustment of the process parameters and with this on the properties of the component is possible.
A complete characterization of the material properties by determining the mechanical characteristic values, tenacity, inherent stress condition, microstructure and texture over the length and width of the strip or the metal sheet and the formed tubes via a feed back to the manufacturing process can currently only be realized for individual metal sheets or strips with destructive testing methods and therefore only with great effort which is economically not feasible.
A complete testing and documentation of the metal sheet/strip quality regarding the material properties is therefore currently not undertaken.
No method is known date with which the material properties can economically be tested over the entire metal sheet or strip surface and with which the properties of the metal sheet/strip for example with regard to the forming behavior during forming of the metal sheet/strip to a open seam tube can be characterized.